Bias current generating circuits are commonly employed in integrated circuit devices in order to generate a bias current from an external power supply voltage. An ideal bias current generating circuit generates a consistent bias current that is independent of variation in applied power, process parameters and temperature.
A conventional bias current generation circuit is disclosed in U.S. Pat. No. 6,201,436, the content of which is incorporated herein by reference. Such a circuit employs a first current generator in which a first generated current is proportional to absolute temperature (PTAT), or increases with increased temperature, and a second current generator in which a second generated current is inverse-proportional to absolute temperature (IPTAT), or decreases with increased temperature. The first and second generated currents are summed to generate a combined bias current with reduced susceptibility to variation in temperature and applied power.
In the conventional design, the PTAT and IPTAT current generators employ a resistor to generate the respective first and second currents. Since resistors are highly susceptible to process variation and operating temperature variation, the resulting bias current in the conventional approach is likewise susceptible to process and temperature variations.